1. Field of Invention
This invention relates to soft and flexible electrical heaters, and particularly to heating elements, which have soft and strong metal or carbon containing electrically conductive textile threads/fibers.
2. Description of the Prior Art
Heating elements have extremely wide applications in household items, construction, industrial processes, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications.
Numerous types of thin and flexible heating elements have been proposed. For example, U.S. Pat. No. 4,764,665 to Orban et al. discloses an electrically heated fabric for use in gloves, airfoils and aircraft parts. In this patent the fabric is metallized after being formed in a glove structure, following weaving or arranging in a non-woven format. Copper bus bars are utilized for introduction of electrical current to the metallized textile. Having been made of a solid piece of fabric with metallized coating, this heating element does not allow for flexibility in selection of desired power density.
Furthermore, the metallizing of the formed heating element results in a loss of significant economies of scale; only a small number of embodiments can be achieved, thus severely limiting the potential application of this invention. The ""665 design is also not conducive to tight hermetic sealing throughout the heater areas (no gaps inside), which can increase the possibility of a short circuit through puncture and admission of liquid into the body of the heating element. In addition, this element cannot be used with higher temperatures due to the damage that would be caused to the polyaramid, polyester or cotton metallized fabric, described in the invention.
Another prior art example is U.S. Pat. No. 4,713,531 to Fennekels et al. Fennekels et al. discloses a sheet textile structure having resistance elements combined with it. These resistance elements comprise metallic fibers or filaments with a denier like that of natural or synthetic textile fibers, and with overall cross sectional thickness of 8 to 24 microns. The ""531 design suffers from the following drawbacks: (a) being a sheet product, it is not conducive to hermetic sealing throughout the body of the heater (no gaps inside); only perimeter sealing is possible, which can increase the possibility of a short circuit due to puncture and admission of liquid into the body of the heating element; (b) yarns, comprising metal fibers, lack consistency of electrical resistance per given length, and their stretching, compression, or both, will result in very wide fluctuations in resistance, thus limiting the use of this technology for embodiments controlled by strict design and where uncontrollable power output and temperature variability are unacceptable; (c) yams are very heavy: from 1 to 7 grams per 1 meter of yarn; (d) the use of silver fibers makes these yams very expensive; (e) individual conductors have a large cross sectional thickness, each having an outer sheath of braided textile or elastomer.
Another prior art example is U.S. Pat. No. 4,538,054 to de la Bretoniere. The heating element of de la Bretoniere ""054 suffers from the following drawbacks: its manufacturing is complex requiring weaving of metal or carbon fibers into non-conductive fabric in a strictly controlled pattern; the use of the metal wire can result in breakage due to folding and crushing and the use of metal wires affects the softness, weight and flexibility of the finished heater; it can not be manufactured in various shapes, only a rectangular shape is available; only perimeter sealing is possible (no gaps inside), which can result in a short circuit due to puncture and admission of a liquid into the body of the heating element; the method of interweaving of wires and fibers does not result in a strong heating element, the individual wires can easily shift adversely affecting the heater durability; the fabric base of the heating element is flammable and may ignite as a result of a short circuit; it is not suitable for high temperature applications due to destruction of the insulating weaving fibers at temperatures exceeding 120xc2x0 C.
A heating element proposed by Ohgushi (U.S. Pat. No. 4,983,814) is based on a proprietary electro conductive fibrous heating element produced by coating an electrically nonconductive core fiber with electro conductive polyurethane resin containing the carbonatious particles dispersed therein. Ohgushi""s manufacturing process appears to be complex; it utilizes solvents, cyanides and other toxic substances. The resulting heating element has a temperature limit of 100xc2x0 C. and results in a pliable but not soft heating element. In addition, polyurethane, used in Ohgushi""s invention, when heated to high temperature, will decompose, releasing very toxic substances, such as products of isocyanides. As a consequence, such heating element must be hermetically sealed in order to prevent human exposure to toxic off gassing. Ohgushi claims temperature self limiting quality for his invention, however xe2x80x9cactivationxe2x80x9d of this feature results in the destruction of the heater. He proposes the use of the low melting point non-conductive polymer core for his conductive fabric-heating element, which should melt prior to melting of the conductive layer, which uses the polyurethane binder with the melting point of 100xc2x0 C. Thus, the heating element of Ohgushi""s invention operates as Thermal Cut Off (TCO) unit, having low temperature of self destruction, which limits its application.
U.S. Pat. No. 4,149,066 to Niibe et al. describes a sheet-like thin flexible heater made with an electro-conductive paint on a sheet of fabric. This method has the following disadvantages: the paint has a cracking potential as a result of sharp folding, crushing or punching; the element is hermetically sealed only around its perimeter, therefore lacking adequate wear and moisture resistance; such an element can""t be used with high temperatures due to destruction of the underlying fabric and thermal decomposition of the polymerized binder in the paint; the assembly has 7 layers resulting in loss of flexibility and lack of softness.
The present invention seeks to alleviate the drawbacks of the prior art and describes the fabrication of a heater comprising metal fibers, metal wires, metal coated, carbon containing or carbon coated threads/fibers, which is economical to manufacture; does not pose environmental hazards; results in a soft, flexible, strong, thin, and light heating element core, suitable for even small and complex assemblies, such as hand wear. A significant advantage of the proposed invention is that it provides for fabrication of heaters of various shapes and sizes with predetermined electrical characteristics; allows for a durable heater, resistant to kinks and abrasion, and with electro-physical properties that are almost unaffected by application of pressure, sharp folding, small perforations, punctures and crushing. A preferred embodiment of the invention consists of utilizing electrically conductive textile threads/fibers having a Thermal Cut Off (TCO) function to prevent overheating and/or fire hazard. The heaters described in this invention may also comprise a continuous temperature sensor to control heating power output in the heating product.
The first objective of the invention is to provide a significantly safe and reliable heater which can function properly after it has been subjected to sharp folding, kinks, small perforations, punctures or crushing, thereby solving problems associated with conventional flexible metal wire heaters. In order to achieve the first objective, the heater of the present invention may comprise (A) electrically conductive threads/fibers and (B) multi-layer insulation of the conductive threads/fibers. The conductive threads/fibers may be comprised of carbon, metal fibers, textile threads coated with one or combination of the following materials: metal, carbon and/or electrically conductive ink. The proposed heater may also comprise metal wires and their alloys. The electrically conductive textile threads/fibers may possess the following characteristics: (i) high strength; (ii) high strength-to-weight ratio; (iii) softness and flexibility. The beating element core described in this invention is comprised of electrically conductive tapes, sleeves, sheets or cables, which radiate a controlled heat over the entire heating core surface. The multi-layer insulation of the electrically conductive threads/fibers provides increased dielectric properties, preventing or minimizing current leakage in the event of abuse of the heater. The multi-layer insulation may be applied in the form of encapsulation (through extrusion process) or lamination with insulating synthetic materials, having similar or different thermal characteristics.
A second objective of the invention is to provide maximum flexibility and softness of the heating element. In order to achieve the second objective, the electric heating element of the invention may contain thin (0.01 to 3.0 mm, but preferably within the range of 0.05-1.0 mm) conductive threads/fibers, which are woven, non-woven, knitted or stranded into continuous or electrically connected tapes, sleeves/pipes, cables or sheets. Another preferable configuration may consist of extruding soft insulating material, such as, but not limited to polyvinyl chloride (PVC), polyurethane, nylon, polypropylene, temperature resistant rubber, cross-linked PVC or polyethylene around a multitude of electrically conductive textile thread/fibers.
A third objective of the invention is to provide for the uniform distribution of heat, without overheating and hot spots, thereby preventing excessive insulation and improving energy efficiency. In order to achieve this objective (a) conductive threads in the heating elements may be separated by non-conductive fibers/yams or insulating polymers, (b) one side of the heating element may include a metallic foil or a metallized material to provide uniform heat distribution and heat reflection. It is also preferable that the soft heating elements of the invention are made without thick cushioning insulation, which slows down the heat delivery to the surface of the heating unit.
A forth objective of the invention is to provide for ease in the variation of heating power density, thereby solving a problem of manufacturing various heating devices with different electric power density requirements. In order to achieve the forth objective, the electrically conductive textile threads/fibers or metal wires may be insulated by a polymer, creating multiple thin cables, which then may be laminated or sandwiched between woven or non-woven fabric or sheets, knitted, or interwoven with nonconductive threads into heating strips, sleeves/pipes or sheets with predetermined width, density of weaving and thickness. It is preferable that the heating strips and sleeves/pipes, sheets are made of a combination of threads/fibers with different electrical resistance and which may include electrically nonconductive high strength polymer or inorganic (such as refractory ceramic or fiberglass) fibers.
A fifth objective of the invention is to provide a high level of temperature control. In order to achieve the fifth objective, the following materials may be applied: (A) at least one electrically conductive textile thread or fiber, running throughout the heater acts as a temperature sensor and when connected to an electronic power control regulator, activates such electronic power control regulator by a change of electrical resistance or current; (B) at least one fiber optical filament, running throughout the heater acts as an infrared wave temperature sensor. Such infrared wave temperature sensor is connected to an electronic device, which converts infrared wave signals into electrical signals. These electrical signals trigger a power control regulator to adjust or terminate its electrical power output; (C) a power control regulator is set for periodic ON/OFF cycling to minimize temperature build-up in the heater; (D) the power control regulator is set to a predetermined power level and/or power shut-off in a predetermined period of time.
A sixth objective of the invention is to provide a high level of safety, minimizing the possibility of fire hazard. In order to achieve the sixth objective: (A) multiple thin heating cables may be reinforced by strong and flame retardant threads/fibers, (B) the conductive media of the heating cables may comprise metal or carbon containing electrically conductive textile threads/fibers with polymer base having a melting temperature t from 120xc2x0 C. to 350xc2x0 C. The melting of the conductive threads/fibers causes terminating of electrical continuity in the heating system. Thus the proposed heating cables can operate as a high temperature melting fuse or TCO (Thermal-Cut-Off) device, and (C) the electrically conductive threads/fibers, having a melting temperature from 120xc2x0 C. to 350xc2x0 C., may be encapsulated by at least two layers of insulation, providing that the outer insulation layer has a higher melting temperature than the inner insulating layer(s). Use of the outer insulating layer with a higher melting temperature prevents melting of the outer insulation in the event the electrically conductive textile threads/fibers reach their melting temperature (between 120xc2x0 C. to 35xc2x0 C.), thereby terminating electrical continuity in the heater.
A seventh objective of the invention is to provide simple and efficient control of the power output of the heating element. In order to achieve the seventh objective, the heating element, such as heating tape, may be comprised of multiple cables electrically connected in parallel to a power control switch. Variation in the power output may be achieved by switching between one cable to a combination of two or more heating cables.
The present invention comprises a heating element containing soft, strong and light electrically conductive textile threads/fibers acting as a heating means. The heating element is highly resistant to punctures, cuts, small perforations, sharp folding and crushing. It can be manufactured in various shapes and sizes, and it can be designed for a wide range of parameters, including but not limited to input voltage, temperature, power density, type of current (AC or DC) and method of electrical connection (parallel or in series). The heating element preferably consists of non-conductive fibers/yarns or insulating polymers which are combined with electrically conductive individually insulated metal or carbon containing threads/fibers by knitting, weaving into or, laminating between layers of woven or non-woven fabric or sheeting, forming tapes, sleeves/pipes or sheets.
Selected areas of the heating element core may contain electrically conductive textile fibers, fiber optical filaments, or wires to provide sensing of electrical resistance or heat radiation. The heating element core may be shaped by folding, turning, stitching, fusing, laminating or by any other appropriate assembling technique to obtain the predetermined configuration of the heater. The electrical terminals, such as connector pins, crimps or electrodes may be attached to ends of said heating element core. The electrically conductive textile fibers may be electrically connected in parallel or in series. In the event of utilizing alternating current, the individually insulated cables in the heating element core may be connected in such a way as to minimize electromagnetic field (EMF). The following are some of the methods for reducing/eliminating EMF in the preferred embodiments of the invention:
(a) Utilizing of a voltage step-down transformer;
(b) Utilizing of a voltage step-down transformer and rectifier;
(c) Utilizing of an AC to DC rectifier with or without filtering capacitor;
(d) Providing of simultaneous opposite current flow in the individually insulated cables of the heating element.